In the earlier U.S. Pat. of Albert R. Uhlig, No. 3,767,747, there has been proposed a method of blow molding plastic articles by means of a two-step blowing operation. By this earlier invention, a parison is blown into a pre-form, and then the blown pre-form is blown into a final article. This two-stage blowing operation greatly increases the efficiency of blow molding machines of conventional type, and this two-stage blowing operation also is particularly adapted to the final blowing of a bi-axially oriented article. All of these advantages of the earlier method are set forth in the above-identified Uhlig patent. However, in this earlier patent, both the pre-form blow mold and the final blow mold were shifted toward and away from the parison-forming location. The shifting of this large mold mass poses substantial machine design and construction difficulties. Such difficulties particularly were involved in the utilization of multiple cavity molds or molds for large articles, such as gallon containers.
Additionally, the earlier method and apparatus wherein both molds were shifted required the utilization of different blow pins for the two blowing operations, and there was no continuous cooling contact between a single blow pin and the finish or neck of a container, for example. Further, the method and apparatus disclosed in the earlier Uhlig patent immediately transferred the blown pre-form into the final blow mold. The blown pre-form was not cooled in the air in any intermediate station, nor during any appreciable transport time between molds.
In summary, the method and apparatus of Uhlig patent is a practical, workable method and apparatus for blow molding. However, the method and apparatus do not take the maximum advantage of the two-step blowing operation. The present invention proposes certain improved methods and apparatus for fully utilizing all of the thermal and mechanical advantages of the earlier patented concepts of Uhlig.